1. Field of the Invention
This invention relates to doctor blades used in various applications, including cleaning, creping and coating in paper making, tissue making, web converting, and similar operations.
2. Description of the Prior Art
Doctor blades contact the surfaces of rolls in paper making, tissue making and web converting machines for the purpose of cleaning, applying coatings to sheets, or sheet removal. Conventional doctor blade materials include metals, homogeneous plastics, and composite laminates made of synthetic and natural fibers.
Conventional doctor blades typically have a monolithic edge to edge structure. Selection of blade material therefore entails striking a compromise between materials which provide adequate resistance to edge wear, and materials having the tensile and yield strengths necessary to operate effectively in the intended doctoring mode. Often, this necessity to compromise results in the selection of a blade material with less than optimum resistance to edge wear.
There are numerous doctoring processes where blade edge wear can be particularly problematic. For example, in creping and coating, the quality of the resulting paper product is directly affected by the geometry of the blade edge. As the blade wears and the geometry changes, product characteristics such as bulk, tensile strength, softness or crepe count are adversely affected.
In cleaning operation, blade loading is directly related to the contact area of the blade edge. As the blade wears, its contact area increases with a concomitant reduction in contact pressure. Lower contact pressures can reduce cleaning effectiveness, which in turn can produce holes in the sheet, sheet breaks and/or sheet wraps.
In the past, those skilled in the art have sought to avoid or at least minimize the above problems by resorting to more frequent blade changes. However, this too is disadvantageous in that it reduces the overall efficiency of the paper making process.
Other attempts at extending blade life have included hardening blade surfaces by means of an ion nitriding process, as described in U.S. Pat. No. 5,753,076 (King et al.), or employing ceramic wear strips as disclosed in U.S. Pat. No. 5,863,329 (Yamanouchi). A number of drawbacks are associated with ion nitriding processes, including inter alia, high capital investments for costly vacuum chambers, batch processing of individual blades as opposed to the more economical processing of long lengths of coiled blade stock, and the uncontrolled application of the process to all blade surfaces rather than to only the edge regions which are susceptible to wear, which further increases costs.
Although ceramic wear strips beneficially extend blade life, their extreme hardness can produce excessive wear of certain roll surfaces, in particular the cast iron surfaces of yankee rolls. This in turn necessitates frequent and costly roll regrinding. Ceramic tipped blades penetrate much deeper into roll coatings, making it necessary to reduce blade loading pressures by as much as 30%. In creping operations, this reduced loading can have a detrimental effect on tissue properties. Ceramic materials are also expensive and as such, add significantly and disadvantageously to high blade costs.
The principal objective of the present invention is the provision of an improved doctor blade which has greater resistance to edge wear, thus providing a more consistent blade geometry, which in turn improves the quality and consistency of the paper products being produced. Greater resistance to blade wear also increases the overall efficiency of the paper making process by reducing the frequency of blade changing.
A doctor blade in accordance with the present invention has a steel support band configured with a width and thickness suitable for mounting in a blade holder, with tensile and yield strengths suitable for the intended doctoring application. A wear resistant strip of high-speed steel is integrally joined to an edge of the support band, preferably by electron beam welding. The wear resistant strip has tensile and yield strengths higher than those of the support band, with a hardness of between about 55 to 75 Rc.
These and other features and advantages of the present invention will now be described in greater detail with reference to the accompanying drawings, wherein: